The present invention relates to a vacuum braking force booster having a sequential control valve without response path.
Vacuum braking-force boosters are shown, for instance, in Federal Republic of Germany Pat. No. 12 61 411. In that case, the opening or closing of the sequential control valve is controlled by a piston rod which is connected to a sleeve which is displaceable within the diaphragm disk. The piston rod is mechanically coupled to the brake pedal. The working position is obtained by the displacement of the sleeve. But, boosting commences only when the inner valve seat of the equalizing valve, which seat is arranged on the diaphragm disk, is seated, after overcoming the valve closure path, against the valve disk and then the outer valve seat of the control valve, which seat is arranged in the sleeve, lifts off from the valve.
In order to improve this response, it is generally desirable to minimize the valve closure paths in order to eliminate the idle path of the brake pedal which results.
Attempts at solving this problem are already known from Japanese Pat. No. 57-198 158A and from Federal Republic of Germany Unexamined Application for Patent DE-OS No. 31 14 965. In that case, the unit which consists of the control valve piston and piston rod is provided on the control valve piston with a transverse bolt which penetrates through the piston or surrounds it and protrudes beyond the diameter of the piston neck of the booster piston. During return travel after a working stroke, the control valve piston lifts off with the bellows seal from the equalizing valve on the booster piston and by means of the transverse bolt, the control valve piston comes to a standstill and stops against the housing in front of the booster piston.
The valve clearance present during the return travel between the bellows seal and the equalizing valve seat on the booster piston is bridged over by the booster piston, which is pushed further by the return spring. Adaptation of the differential pressure between the rear and front chambers simultaneously takes place until the outer equalizing valve seal rests against the bellows seal. The arrangement of a transverse bolt having the function described above, however, requires an opening through the piston neck of the booster piston. This results in a reduction in the load-bearing cross sections on the booster piston and has an unfavorable effect on the strength of the part when the vacuum braking-force is acted on by the forces of reaction produced by the compressed reaction disk. Ordinary part tests, such as maximum extraction resistance without loss of function between control valve piston with the piston rod supported therein, which apply just as well for the transverse bolt which is connected with the control valve piston and lies on the housing, require a strong profiling and large cross sections of the booster piston. Furthermore, as a result of this, the cross sections of the flow channels passed through by the transverse bolt between the two chambers in the neck of the piston are reduced, which also has an unfavorable effect on the response and control behavior of the apparatus.